1. Field of Invention
This invention relates generally to optical information storage systems and, more particularly, to the read/write heads or actuators providing the interaction with the optical information storage medium. The actuator supplies the radiation beam which is applied to the storage medium and/or detects radiation which has interacted with the storage medium. The actuator must insure that the radiation beam accesses (or tracks) the correct region of the storage medium. In addition, the actuator must control the focus of the radiation beam on the optical storage medium.
2. Description of the Related Art
In the optical information storage system, the actuator or read/write head applies a radiation beam to and/or detects a radiation beam that has interacted with the optical storage medium. The actuator, which typically includes an objective lens, can have six degrees of freedom. These degrees of freedom are used to manipulate the applied radiation beam and include three translational degrees of freedom and three rotational degrees of freedom. As will be clear, the rotational degree of freedom about the optic axis, the axis defining the preferred path of the radiation beam, is generally irrelevant. For tracking control, the radiation beam impinging on the storage medium must be controlled in a two dimensional plane for a flat storage medium surface (i.e., in the implementation involving an optical disc, the tracking is typically accomplished by motion along the radius of a disc). For focus adjustment, the distance from a radiation beam focusing component, e.g., an objective lens, to the storage medium must be controlled. A description of the parameters of important in tracking and focussing of an actuator element, are discussed in OPTICAL RECORDING, A Technical Overview by Alan B Marchant, Addison-Wesley Publishing Company, 1990.
As indicated in the Marchant reference, the radiation beam which has interacted with the optical storage media can be processed in such a manner as to yield a signal (or signals) related to focus control and a signal (or signals) related to tracking control. These signals can be used as part of feedback loop to control the location and extent of the interaction of the radiation beam with the storage medium. However, the requirements on the focus and tracking control have diverse operating requirements. As a result of the requirements, both a coarse and a fine adjustment can be required. For example, the tracking control system must not only provide a gross tracking mechanism to position the read/write head over the appropriate track region on the storage medium and over a selected track, but the tracking control system must compensate for any deviation of the track from concentric circle or spiral (relative to the spindle of a disc storage medium). Similarly, not only must the optical read/write head position the objective lens an appropriate distance from the storage medium, but, the focus system must continuously adjust the focus not only for minor departures from a planar surface, but for variations in the thickness of the dielectric layer protecting the storage surface of the medium. Providing the appropriate adjustments for the minor corrections in the focussing and tracking control can be difficult because, in implementing the adjusting movements, the entire inertia of the read/write head must be overcome.
A need has therefore been felt for a technique and associated apparatus for providing fine focus and tracking adjustments for the read/write head of an optical disk of an optical storage and retrieval system which do not require positioning of the entire head.